Much is made in Free Software/Open Source discussions
of the "Right to fork", which is a fundamental element of any true
Free or Open Source licence. It is one of things that the Open
Source Initiative (OSI) and the Free
Software Foundation (FSF) agree on.
The biggest disagreements amongst open/free proponents
in the "Viral" or "Copyleft" feature of the GPL,
which prevents anyone from making a fork proprietry. Now, this particular
feature of the GPL seems to cause the most confusion. So, here's my attempt
to ease the confusion.
Copyleft is the "right to merge".
This is the unique feature of the GPL. Other free/open
licences grant a right to fork, but GPL makes sure that any forks can later
be merged, that good contributions can be used to make all forks better,
rather than any particular fork.
This is an interesting point, I think, because,
where the other elements of the GPL are there for the benefit of programmers,
this particular feature is there for the benefit of the code, and therefore
the end-user. In the end, that's what the GPL is all about, making sure
that users can get the best quality code.